Add delete-downloaded-video and real YouTube unsubscribe

Delete local copy:
- MeTubeClient.delete_download() -> POST /delete {ids, where: "done"}
- Only ever acts on a job our own app tracked (metube_job_id we stored from
  its own 'completed' event), never a pre-existing MeTube file
- New "deleted" terminal status; DELETE /api/videos/{id}/download
- Frontend: delete button next to "На сервере" badge, confirm dialog

Unsubscribe (deliberate deviation from the original TZ's MVP exclusion of
subscription management, per explicit user request after being shown the
tradeoff):
- OAuth scope widened from youtube.readonly to full youtube (read/write) --
  existing stored tokens only cover the old scope, so unsubscribing needs a
  fresh reconnect; reads keep working unchanged on the old token meanwhile
- channels.youtube_subscription_id (distinct from the channel id; that's
  what subscriptions.delete actually keys on) captured during subscriptions
  sync
- YouTubeInsufficientScope raised on 401/403 "insufficient authentication
  scopes" and surfaced as a clear 403 asking the user to reconnect, rather
  than a generic API error
- POST /api/channels/{id}/unsubscribe calls subscriptions.delete and marks
  the channel unsubscribed locally on success
- Frontend: "Отписаться" button on ChannelCard with confirm dialog

10 new backend tests (73 total).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/DownloadButton.tsx

@@ -1,7 +1,7 @@
 import { useEffect, useRef, useState } from 'react'
 import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query'
 import type { FeedVideoDto } from '../api/client'
-import { downloadVideo, getDownloadStatus } from '../api/client'
+import { deleteDownload, downloadVideo, getDownloadStatus } from '../api/client'
 
 const ACTIVE_STATUSES = ['queued', 'downloading', 'postprocessing']
 
@@ -40,6 +40,13 @@ function DownloadButton({ video }: Props) {
     },
   })
 
+  const deleteMutation = useMutation({
+    mutationFn: () => deleteDownload(video.youtube_video_id),
+    onSuccess: (data) => {
+      queryClient.setQueryData(['download-status', video.youtube_video_id], data)
+    },
+  })
+
   const status = statusQuery.data?.status ?? video.local.status
   const percent = statusQuery.data?.progress_percent ?? video.local.progress_percent
 
@@ -47,7 +54,7 @@ function DownloadButton({ video }: Props) {
   useEffect(() => {
     if (prevStatus.current !== status) {
       prevStatus.current = status
-      if (status === 'completed' || status === 'failed') {
+      if (status === 'completed' || status === 'failed' || status === 'deleted') {
         queryClient.invalidateQueries({ queryKey: ['feed'] })
         queryClient.invalidateQueries({ queryKey: ['video', video.youtube_video_id] })
       }
@@ -55,7 +62,21 @@ function DownloadButton({ video }: Props) {
   }, [status, queryClient, video.youtube_video_id])
 
   if (status === 'completed') {
-    return <span className="download-badge completed">✓ На сервере</span>
+    return (
+      <span className="download-inline-actions">
+        <span className="download-badge completed">✓ На сервере</span>
+        <button
+          onClick={() => {
+            if (confirm('Удалить скачанную копию с сервера?')) {
+              deleteMutation.mutate()
+            }
+          }}
+          disabled={deleteMutation.isPending}
+        >
+          Удалить
+        </button>
+      </span>
+    )
   }
 
   if (status === 'downloading') {